Russia's foreign minister said Friday that Syria's moderate opposition must separate from certain terrorist groups before there can be a real cessation of hostilities.

"So we are all in favor of a cease-fire, but without separation of the opposition from Nusra, the cease-fire is meaningless," Sergei Lavrov said at a news conference on the sidelines of the U.N. General Assembly in New York.

"Unfortunately, the coalition, led by the United States, which committed itself to make sure that this separation happens, has not been able to do this," he added.

In July, the group known as Jabhat al-Nusra said it was severing ties with al-Qaida and rebranding itself as Jabhat Fatah al-Sham.

Lavrov complained that since the United States and Russia, as co-chairs of the International Syria Support Group, agreed on the truce and it went into effect September 12, there have been by his count almost 350 opposition-led attacks in Aleppo alone.

'Unholy alliance'

U.S. Secretary of State John Kerry said earlier this week that some elements of the Syrian opposition have relied for too long "on an unholy alliance" with Nusra, and that "we cannot look the other way" if they are fighting on the ground with them.

Kerry also called for grounding all aircraft in key parts of Syria to de-escalate the situation and allow humanitarian assistance to be delivered. He said if that happened, the fraying cease-fire might be revived.

In his address Friday to the U.N. General Assembly, Lavrov said Russia's military assistance prevented the collapse of Syria and the disintegration of that country "under the onslaught of terrorists."

"The Syrian crisis would not be resolved and the appalling humanitarian situation would not be rectified without suppressing ISIS, Jabhat al-Nusra and their associate extremist groups," Lavrov said, using an acronym for Islamic State. "This is a key condition for strengthening the cessation-of-hostilities regime and overall national truce."

The top American and Russian diplomats have been meeting daily this week in New York on the sidelines of the U.N. annual meetings, but they have failed to make any significant progress on Syria.